The reduction of undesired emissions from internal combustion engines is commonly provided by the use of a catalytic converter in the exhaust gas path of the engine. In a two-way catalytic device, the catalyst is effective to cause oxidation of carbon monoxide and hydro-carbons in the exhaust gases. In a so-called three-way catalytic device, the catalyst also causes the reduction of nitrogen oxides. In the two-way catalytic device, the efficiency of the converter increases with increasing air fuel ratio in the region of the stoichiometric value i.e. the ratio at which the fuel and oxygen are proportional so that both would be completely consumed in perfect combustion. In the case of the three-way catalytic device, the optimum conversion efficiency for combined oxidation and reduction is obtained when the air-fuel ratio is maintained within a narrow range centered at the stoichiometric value. Preferably, for reduction of emissions, the air-fuel ratio should be maintained on the lean side of the stoichiometric value with a two-way catalytic device and with a three-way device, within a narrow band or window centered on the stoichiometric value.
Numerous control systems have been proposed for controlling the air-fuel ratio supplied to the engine in response to a measure of the composition of the exhaust gases. A typical arrangement uses a closed loop control system with an oxygen gas sensor in the exhaust gas passage for producing a control signal for modifying the air-fuel ratio. Presently known oxygen gas sensors, such as the zirconium oxide type sensor, are responsive to produce an electrical signal which accurately represents an oxygen content only if the gas sensor is above a certain temperature. When an engine is warming up after a cold start or when operating during a prolonged idle condition, the oxygen sensor will be below it's operating temperature. In this condition the closed loop control system with the oxygen sensor is unable to control the air-fuel ratio at the desired value. It has been proposed to overcome this problem by disabling the closed loop control system until the oxygen sensor has reached it's operating temperature. In the Oberstadt U.S. Pat. No. 3,938,479 the operating condition of the oxygen sensor is detected by monitoring the output signal and the closed loop control system using the oxygen sensor becomes operative only when the oxygen sensor is in an operative state. A system is disclosed in the Toelle et al U.S. Pat. No. 3,990,411 in which the closed loop system is operated to maintain the air-fuel ratio at a fixed or predetermined value for given engine operating conditions until the oxygen sensor is at the proper operating temperature.
While such prior art systems are highly effective for controlling the air-fuel ratio after the oxygen sensor becomes operative, there is a need for improved control of the air-fuel ratio during the warm-up period or during cold engine operation. There have been efforts to provide control of the air-fuel ratio during engine warm-up by using one control system and to provide control after engine warm-up by means of another control system. In the Williams U.S. Pat. No. 3,926,154, one control loop uses a carbon monoxide sensor and controls the air-fuel ratio durng engine warm-up and another control loop uses an oxygen sensor to control the air-fuel ratio after warm-up. In the Storey U.S. Pat. No. 4,027,477, a closed loop control system is disclosed with an oxygen sensor upstream of the catalytic converter and another oxygen sensor downstream of the catalytic converter. The first oxygen sensor reaches operating temperatures more quickly after engine start-up than the second sensor and means are provided to transfer control from the first sensor to the second sensor after the second sensor reaches operating temperature.
It is a general object of this invention to overcome the disadvantages of the prior art by providing alternately selectable closed loop control system for the air-fuel ratio during engine operation.